1. Field of the Invention
The invention relates to thin-film solar cells and method of making.
2. Background Information
Photovoltaic (PV) cells are made of materials referred to as semiconductors, such as, silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy impacts the electrons, allowing them to flow freely. PV cells also all have one or more electric fields which act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, one can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell""s voltage (which is a result of its built-in electric field or fields), defines the power that the solar cell can produce.
A display screen made with TFT (thin-film transistor) technology is a liquid crystal display (LCD), common in notebook and laptop computers, that has a transistor for each pixel (that is, for each of the tiny elements that control the illumination of your display). Having a transistor at each pixel means that the current that triggers pixel illumination can be smaller and therefore can be switched on and off more quickly. TFT technology is also known as active matrix display technology (and contrasts with xe2x80x9cpassive matrixxe2x80x9d which does not have a transistor at each pixel). A TFT or active matrix display is more responsive to change. For example, when you move your mouse across the screen, a TFT display is fast enough to reflect the movement of the mouse cursor. (With a passive matrix display, the cursor temporarily disappears until the display can xe2x80x9ccatch up.xe2x80x9d) Active matrix (also known as Thin Film Transistor or thin film transistor) is a technology used in the flat panel liquid crystal displays of notebook and laptop computers. Active matrix displays provide a more responsive image at a wider range of viewing angle than dual scan (passive matrix) displays.
In this context, an Si:H film is a film of silicon in which hydrogen is incorporated. The hydrogen content is approximately 3 to 20%.
Solar cells based on the semiconductor material silicon have been known for many years. These solar cells are usually produced from solid monocrystalline or polycrystalline silicon, typical thicknesses of a solar cell of this type being approximately 300 to 500 xcexcm. These thicknesses are required firstly in order to ensure sufficient mechanical stability and secondly to achieve absorption of the incident sunlight which is as complete as possible. On account of the relatively large film thicknesses and the associated high consumption of material, and on account of the unavoidable need for a high-temperature step for doping of the silicon wafers (Txe2x89xa71000xc2x0 C.), solar cells of this type entail expensive manufacture.
As an alternative to these relatively thick silicon solar cells described above, in addition to the thin film solar cells based on amorphous Si:H (referred to below as a-Si:H), which have already been the subject of research for some 20 years, thin-film solar cells made from microcrystalline Si:H (referred to below as xcexcc-Si:H) have in recent years become an established subject for investigation. This cell material is expected to have a similarly high efficiency to that of monocrystalline silicon, but to involve less expensive production processes, as are also known for a-Si:H. At any rate, the use of xcexcc-Si:H is supposed to suppress the degradation in the efficiency under intensive illumination, which is inevitable when using a-Si:H. However, a number of significant points still currently stand in the way of commercial utilization of xcexcc-Si:H as the functional layer in a thin-film solar cell. Unlike the solar cell using a-Si:H, which has a thickness of the photovoltaically active film of approximately 300 nm, the solar cell made from xcexcc-Si:H, to achieve a similarly good utilization of the incident light, must be approximately 3000 nm thick, i.e. has to be thicker by a factor of 10. Therefore, an economic process must also allow the deposition rate of the microcrystalline material to be higher by this factor than that achieved for a-Si:H. An inexpensive substrate, preferably window glass or even standard plastics, appears to be indispensable as a further necessary feature for commercial utilization of the xcexcc-Si:H. For this purpose, it is necessary to have available deposition methods which are compatible with the substrates, i.e. low-temperature processes (T less than 100xc2x0 C. for plastic or Txe2x89xa6200 to 300xc2x0 C. for glass which is provided with a transparent conductive film), and these processes must moreover still achieve high film-generation rates.
According to the prior art, microcrystalline silicon (xcexcc-Si:H) can be applied in thin films to a support material at temperatures of greater than approximately 200xc2x0 C. using various processes. For example, it can be deposited directly from the gas phase. By way of example, the following deposition methods are known: high-frequency glow discharge deposition (HF-PECVD), electron cyclotron resonance (ECR) process, electron cyclotron wave resonance (ECWR) process, sputter deposition, hot-wire (HW) technique, microwave CVD.
Furthermore, processes are also known in which xcexcc-Si:H is produced by initially depositing a-Si:H from the gas phase, which is then transformed into xcexcc-Si:H. The transformation of a-Si:H to xcexcc-Si:H is known, for example, from the following documents.
For example, U.S. Pat. No. 5,470,619 describes the transformation of a-Si:H into xcexcc-Si:H by means of heat treatment at a temperature of 450xc2x0 C. to 600xc2x0 C.
U.S. Pat. No. 5,486,237 describes a temperature-induced transformation of a-Si:H films into xcexcc-Si:H films at 550xc2x0 C. to 650xc2x0 C. over a period of 3 to 20 hours.
U.S. Pat. No. 5,344,796 describes a process for producing a thin xcexcc-Si:H film on a glass substrate. In this process, first of all a xcexcc-Si:H film is generated on the substrate and serves as a seed layer, then a-Si:H is deposited on this seed layer by means of a CVD process. The a-Si:H is transformed into xcexcc-Si:H by means of a heat treatment, preferably at between 580xc2x0 C. and 600xc2x0 C. for a period of from 20 to 50 hours.
U.S. Pat. No. 5,693,957 likewise describes the thermal transformation of a-Si:H films into xcexcc-Si:H films at 600xc2x0 C., the transformation of certain a-Si:H films into xcexcc-Si:H being deliberately prevented by impurities formed by these a-Si:H films.
A microwave plasma CVD process for the production of a-Si:H and xcexcc-Si:H films is described in U.S. Pat. No. 5,334,423, in which, in saturation mode, 100% of the microwave power is introduced.
Published International Application No. 93/13553 (corresponding to U.S. Pat. No. 5,231,048) describes a microwave CVD process for producing thin semiconductor films, the process pressure lying below the Paschen minimum. A microwave CVD process with controllable bias potential for the production of thin semiconductor films is described in document U.S. Pat. No. 5,204,272.
The production of xcexcc-Si:H films by means of a microwave CVD process is described in U.S. Pat. No. 4,891,330, in which preferably at least 67% of hydrogen is added to the process or precursor gas in order to assist the formation of the xcexcc-Si:H phase.
A plasma process for the production of a xcexcc-Si:H layer is described in document Published International Application No. 97/24769 (corresponding to U.S. Pat. No. 6,309,906), the precursor gas being diluted with hydrogen and/or argon.
Furthermore, a plasma treatment of an a-Si:H film by means of an argon plasma is described in U.S. Pat. No. 4,762,803, and by means of a hydrogen plasma in Published International Application No. 93/10555 (corresponding to U.S. Pat. No. 5,387,542), in order to obtain a xcexcc-Si:H film.
European Patent No. 0 571 632 A1 (corresponding to U.S. Pat. No. 5,387,542) has disclosed a plasma CVD process for producing a microcrystalline Si film on a substrate. For this purpose, firstly a thin, amorphous Si:H film is produced on the substrate by plasma-assisted CVD coating. Then, the amorphous Si:H film is subjected to a plasma-assisted treatment using a hydrogen plasma, the amorphous Si:H film being transformed into the microcrystalline Si:H film.
Plasma-enhanced CVD coating in pulsed mode for the production of an amorphous Si:H film on a substrate is known from U.S. Pat. No. 5,618,758.
Furthermore, it is also possible to produce a xcexcc-Si:H film by alternating deposition of a-Si:H films and subsequent treatment of this film using a hydrogen plasma. This process is generally referred to in the literature as the layer-by-layer (LBL) technique. The process by which the a-Si:H is transformed into xcexcc-Si:H at atomic level has not to date been unambiguously explained (there are several models under discussion), but a competition process between the etching away of disadvantageous Sixe2x80x94Si bonds and hydrogen-induced restructuring of the network toward the crystalline phase, which is more favorable in energy terms, seems very likely.
Parameters which provide good a-Si:H films, i.e. those which are suitable for components, are often used for the deposition of the a-Si:H film. The thicknesses of the individual films which are reported in the literature typically lie between 1.4 nm and several 10s of nm. On account of this relatively great variation in film thickness, the result is aftertreatment, or post-treatment, times using an H2 plasma which lie in the range from a few seconds to several minutes. The deposition processes used are HF-PECVD processes, in which, on account of the low excitation frequency, the deposition rates are relatively low.
HF-PECVD processes at most achieve maximum deposition rates (film thickness actually deposited divided by the time required for this deposition) which are significantly below 10 nm/min.
The following text provides literature references which represent the prior art of xcexcc-Si:H deposition by means of the LBL technique:
Asano, A.; Appl. Phys. Lett. 56 (1990) 533;
Jin Jang; Sung Ok Koh; Tae Gon Kim; Sung Chul Kim, Appl. Phys. Lett. 60 (1992) 2874;
Otobe, M.; Oda, S.; Jpn. J. Appl. Phys. 31 (1992) 1948;
Kyu Chang Park, Sung Yi Kim; Min Park; Jung Mok Jun; Kyung Ha Lee; Jin Jang; Solar Energy Materials and Solar Cells, Vol. 34 (1994), 509;
Hapke, P.; Carius, R.; Finger, F.; Lambertz, A.; Vetterl, O.; Wagner H.; Material Research Society Symposium Proceedings, Vol. 452; (1997), 737.
All the processes which have been used to date for the LBL technique give very low effective deposition rates (1-6 nm/min), which restrict commercial application. Furthermore, in the LBL processes which have been used to date, the individual film thicknesses (1 nm to a few 10s of nm) cannot reliably be set with accuracy without a complex in situ measurement technique. This variation from the first step of the process is reflected in the second step. The result in particular is that the duration of the second step (H2 plasma treatment) cannot be determined with accuracy in advance. This means that the process is dependent on an inherent stability which cannot be achieved on an industrial scale.
Measures aimed at increasing the rate, for example by increased introduction of power (higher plasma densities) lead to an increase in the particle fraction in the film and therefore to a reduction in quality.
The literature and the abovementioned documents describe relatively high process temperatures (250-330xc2x0 C.), which are evidently required in order to ensure sufficient film qualities (compact, i.e. dense films) and to ensure film adhesion. Therefore, thermolabile substrates cannot be coated.
In accordance with one object of the invention there is to be provided a solar cell having a low-cost, low thermal stability substrate.
In accordance with another object of the invention there is to be provided a thin-film transistor having a low-cost, low thermal stability substrate.
Working on this basis, the present invention, in at least one aspect, is also based on the object of providing a plasma CVD process and a plasma CVD device for the production of a microcrystalline Si:H film on a substrate, in which the microcrystalline Si:H film is produced by treating an amorphous Si:H film using a hydrogen plasma. The intention is to produce a high-quality microcrystalline Si:H film on a substrate at low cost and with high deposition rates. It is to be possible to set and regulate the film thickness and composition in a controlled manner, and production is to take place with the minimum possible heating of the substrate.
According to one aspect of the invention, there is provided a thin-film solar cell, comprising: a transparent substrate having a first surface configured to receive incident light and a second surface opposite said first surface; a first electrode having a first surface and a second surface opposite said first surface; said first electrode comprising an electrically conductive layer of a transparent conductive material; a microcrystalline hydrogenated silicon semiconductor body; said microcrystalline hydrogenated silicon semiconductor body having a first surface and a second surface opposite said first surface; said microcrystalline hydrogenated silicon semiconductor body being disposed with said first surface thereof on said second surface of said first electrode; said microcrystalline hydrogenated silicon semiconductor body originated from a continuous-gas-flow, pulsed-electromagnetic-radiation-excited plasma, plasma-enhanced chemical vapor deposited, continuous-gas-flow, pulsed-electromagnetic-radiation-excited, hydrogen-plasma-enhanced-treated amorphous hydrogenated silicon body; said second surface of said first electrode comprising a surface configured to accept said microcrystalline hydrogenated silicon semiconductor body; said microcrystalline hydrogenated silicon semiconductor body comprising at least one semiconductor layer; at least one of each said at least one semiconductor layer having a thickness of from about one tenth of a nanometer to about fifty nanometers; a second electrode having a first surface and a second surface opposite said first surface; said second electrode being disposed with said first surface thereof on said second surface of said microcrystalline hydrogenated silicon semiconductor body; a first conductor element connected to said first electrode; and a second conductor element connected to said second electrode; said first conductor element and said second conductor element being configured and disposed to lead electricity from said solar cell; said substrate having a predetermined heat stability; said predetermined heat stability being sufficiently great to permit manufacture of a thin-film solar cell and said predetermined heat stability being sufficiently low to minimize cost.
In accordance with another aspect of the invention there is provided a thin-film transistor, comprising: a substrate having a first surface and a second surface opposite said first surface; a microcrystalline hydrogenated silicon semiconductor body; said microcrystalline hydrogenated silicon semiconductor body having a first surface and a second surface opposite said first surface; said microcrystalline hydrogenated silicon semiconductor body being disposed with said first surface thereof on said second surface of said substrate; said microcrystalline hydrogenated silicon semiconductor body originated from a continuous-gas-flow, pulsed-electromagnetic-radiation-excited plasma, plasma-enhanced chemical vapor deposited, continuous-gas-flow, pulsed-electromagnetic-radiation-excited, hydrogen-plasma-enhanced-treated amorphous hydrogenated silicon body; said microcrystalline hydrogenated silicon semiconductor body comprising at least one semiconductor layer; at least one of each said at least one semiconductor layer having a thickness of from about one tenth of a nanometer to about fifty nanometers; said microcrystalline hydrogenated silicon semiconductor body comprising a source layer and a drain layer; a plurality of insulating films disposed on said microcrystalline hydrogenated silicon semiconductor body; said plurality of insulating films comprising a first insulating film, a second insulating film, and a third insulating film; a gate electrode disposed on said first insulating film; a source electrode disposed on said second insulating film; a drain electrode disposed on said third insulating film; said substrate comprising a predetermined heat stability; said predetermined heat stability being sufficiently great to permit manufacture of a thin-film transistor and said predetermined heat stability being sufficiently low to minimize cost.
In accordance with one aspect of the invention there is provided a process for providing a microcrystalline hydrogenated silicon semiconductor body on a substrate, such as, a substrate for a thin-film solar cell, or a substrate for a thin-film transistor, said process comprising: providing a substrate, said substrate having a first surface and a second surface opposite said first surface; flowing a plasma-enhanced chemical vapor deposition gas over said second surface of said substrate to deposit a body of amorphous hydrogenated silicon on said second surface of said substrate; flowing a plasma-enhanced, hydrogen-plasma containing conversion gas over said deposited body of amorphous hydrogenated silicon to convert said deposited body of amorphous hydrogenated silicon into a body of microcrystalline hydrogenated silicon; said flowing of said deposition gas and said flowing of said conversion gas comprising at least one of: (a.), (b.), (c.), and (d.): (a.) continuously flowing said plasma-enhanced chemical vapor deposition gas over said second surface of said substrate to deposit said body of amorphous hydrogenated silicon on said second surface of said substrate; (b.) continuously flowing said plasma-enhanced, hydrogen-plasma containing conversion gas over said body of amorphous hydrogenated silicon to convert said deposited body of amorphous hydrogenated silicon into a body of microcrystalline hydrogenated silicon; (c.) exposing said plasma-enhanced chemical vapor deposition gas to a pulsed electromagnetic radiation with a sufficient radiation intensity to excite said plasma contained in said plasma-enhanced chemical vapor deposition gas thus depositing said deposited body of amorphous hydrogenated silicon on said second surface of said substrate; (d.) exposing said plasma-enhanced, hydrogen-plasma conversion gas to a pulsed electromagnetic radiation with a sufficient radiation intensity to excite said plasma contained in said plasma-enhanced, hydrogen-plasma conversion gas to thus effectuate conversion of said amorphous hydrogenated silicon body into said deposited body of microcrystalline hydrogenated silicon; and said method further comprising: attaching at least two electrode means to said body of microcrystalline hydrogenated silicon and forming one of: a thin-film solar cell, or a thin-film transistor.
According to one aspect of the invention, to achieve this object, there is proposed a plasma CVD process for the production of a microcrystalline Si:H film on a substrate, comprising the following steps:
1.1 plasma-enhanced CVD coating of the substrate with at least one thin amorphous Si:H film,
1.2 plasma-enhanced treatment of the amorphous Si:H film using a hydrogen plasma, the amorphous Si:H film being transformed into a microcrystalline Si:H film, and
1.3 repeating the steps 1.1 and 1.2 if necessary which is characterized in that the coating or the treatment is carried out with a continuous flow of the coating gases or the treatment gases and using pulsed electromagnetic radiation which excites the plasma.
With regard to the device, the object is achieved, according to one aspect of the invention, by the fact that a device for producing a microcrystalline Si:H film on a substrate using a plasma CVD process is provided, in which an amorphous Si:H film is deposited in pulse-induced manner on the inner surfaces of the device, in particular on the inner surfaces of the deposition chamber.
Plasma impulse CVD processes are known and are described, for example, in Journal of the Ceramic Society of Japan, 99 (10), 894-902 (1991), this document being hereby incorporated by reference as if set forth in its entirety herein. In these processes, generally with a continuous flow of the coating gases, the electromagnetic radiation which excites the plasma is supplied in pulsed form, a thin film (typically of xe2x89xa70.1 nm) being deposited on the substrate on each pulse. The fact that each power pulse is followed by a pulse pause means that even substrates which are not thermally stable can be exposed to high powers during a pulse. This means in particular that high coating rates are possible without imposing significant thermal loads on the substrate.
Therefore, the plasma CVD process according to one aspect of the invention for the first time allows very rapid, inexpensive production of high-quality, microcrystalline Si:H films on a substrate. The film thickness and the composition of the Si:H film can be set and regulated reproducibly. The film is produced with very minor heating of the substrate.
The amorphous Si:H film is preferably deposited in individual film assemblies, it being possible to produce film assemblies comprising 1 to 50, particularly 1 to 5 a-Si:H monolayers per pulse.
The film thickness of a film assembly can in this case be set reproducibly. Under otherwise constant conditions, a defined film thickness of a-Si:H is always deposited on each pulse. It is in this way possible to set a multiple of the film thickness of a film assembly by simply counting the number of pulses. The film thickness of a film assembly can for this purpose be determined experimentally on a one-off basis. In other words, there need to be only one experimental determination of the film thickness of a film assembly.
With a predetermined or gettable film thickness of the a-Si:H film, the pulse-induced treatment duration with the hydrogen plasma can also easily be predetermined experimentally and therefore accurately defined.
After each pulse and therefore deposition of an a-Si:H film assembly, the coating gas is preferably changed very quickly, i.e. the gas is discharged and a new coating gas is passed into the deposition chamber.
The first film layers applied to the substrate are preferably deposited in the form of a degressive gradient with an elevated, inherent microcrystalline Si:H content. A preferred process for the production of a gradient film is described in German Patent No. 44 45 427 C2 (corresponding to U.S. Pat. No. 5,643,638). The fact that the first film already has a certain amount of xcexcc-Si:H means that the subsequent transformation from a-Si:H to xcexcc-Si:H is significantly quicker and easier, since the crystalline formation is present in the first film layers.
This eliminates the need for further gradient films to be produced. Since this procedure is highly time-consuming and complex, after a gradient film containing xcexcc-Si:H has been produced once, the process is switched in such a way that subsequently only a-Si:H is deposited, and this material is transformed into xcexcc-Si:H.
Preferably, in each case a thin, amorphous Si:H film which is from 0.1 to 5 nm thick is deposited and is then transformed into xcexcc-Si:H, with a duration of a pulse of the electromagnetic radiation of xe2x89xa70.1 ms and a pulse pause of the electromagnetic radiationxe2x80x94i.e. the pause between two pulsesxe2x80x94of xe2x89xa6200 ms being set.
The treatment time using the pulsed hydrogen plasma is preferably set at up to 30 seconds, in particular at up to 10 seconds.
Overall, a microcrystalline Si:H film which is up to 5000 nm thick is produced on the substrate; greater thicknesses are possible without any restrictions.
The PICVD process can be carried out using alternating voltage pulses with a frequency of between approximately 50 kHz and 300 GHz. On account of the high coating rate and the possibility of working within a relatively broad pressure range (0.001 to approximately 10 mbar), microwave frequencies are particularly suitable; of these frequencies, the 2.45 GHz frequency is preferred as the industrial frequency, since the corresponding microwave components are readily available at low cost. As a further advantage, the pulse process offers the possibility of shaping the pulse itself, and in this way further influencing properties of the thin film which is deposited by a single plasma pulse in terms of the film growth direction. At a pressure of 0.1-2 mbar, an excitation frequency of 2.45 GHz, pulse durations are between 0.1 and 2 ms and pulse pauses of between 5 and xe2x89xa6200 ms have proven particularly suitable for the production of the types of film according to one aspect of the invention. If the reaction times in the plasma are very short, pulse durations of 0.01 ms may be appropriate; however, the use of such short pulses is often restricted by equipment considerations (pulse rise time). The recommended range for the pulse amplitude cannot be stated numerically; the minimum value is that value at which the discharge can still just be initiated with the particular coating gas and the other process parameters, and the maximum value is given by the capacity of the particular pulse generator used.
The procedure for producing the gradient layer will as a rule be such that the dependence of the layer properties and/or compositions on the pulse duration, pulse amplitude and pulse pause are determined in a series of preliminary experiments and, during the actual production of the gradient film, this parameter is controlled in such a way that the desired gradient is formed in the film growth direction. The accuracy with which the gradient must be fixed beforehand depends on the demands imposed on the layer. With the process according to one aspect of the invention, it is possible without difficulty to vary, for example, the composition of the film on the substrate in the film growth direction from monolayer to monolayer.
A mean microwave power of 150 mW/cm3 to 1500 mW/cm3 is preferably used.
The amorphous Si:H film is preferably deposited from a coating gas which contains at least one Si-organic film-forming agent, the coating gas used being a silane, in particular SiH4 or a chlorosilane, and a process pressure in the range from 0.1 to 1 mbar being set. Even at high deposition rates, i.e. a relatively high process pressure and a high pulse power, contrary to expectation no dust or powder formation was observed in the film.
It is particularly advantageous if the coating gas is changed very quickly after each a-Si:H film. Very rapid gas change times ( less than 10 ms) makes the process particularly economical, and it is possible to reproducibly produce xcexcc-Si:H films of settable thickness and quality.
Hydrogen may be added to the coating gas.
The process according to one aspect of the invention is preferably carried out in such a manner that the substrate temperature does not exceed 200xc2x0 C., in particular 100xc2x0 C., and particularly preferably 50xc2x0 C.
According to the process according to one aspect of the invention, it is advantageously possible to set conductivities of the xcexcc-Si:H film of from 10xe2x88x927 S/cm to 10 S/cm, the conductivities if appropriate being adjusted by doping with foreign atoms, for example by means of the coating gas.
In this case, it is preferable to produce an n-doped, p-doped or undoped xcexcc-Si:H film. Particularly for the production of thin-film solar cells, it is necessary to produce a plurality of different xcexcc-Si:H films on top of one another on a substrate.
The substrate used is preferably a glass, a glass ceramic or a plastic, the substrate particularly preferably being provided with a transparent, conductive film, in particular an ITO film, a doped SnO2 film or a doped ZnO film.
A xcexcc-Si:H film on a substrate which has been produced using the process according to one aspect of the invention is preferably used as a component of a thin-film solar cell or as a component of a thin-film transistor (TFT).
The above-discussed embodiments of the present invention will be described further hereinbelow. When the word xe2x80x9cinventionxe2x80x9d is used in this specification, the word xe2x80x9cinventionxe2x80x9d includes xe2x80x9cinventionsxe2x80x9d, that is the plural of xe2x80x9cinventionxe2x80x9d. By stating xe2x80x9cinventionxe2x80x9d, the Applicants do not in any way admit that the present application does not include more than one patentably and non-obviously distinct invention, and maintains that this application may include more than one patentably and non-obviously distinct invention. The Applicants hereby assert that the disclosure of this application may include more than one invention, and, in the event that there is more than one invention, that these inventions may be patentable and non-obvious one with respect to the other.